The possibilities are limitless or rather limited only by your own thought process. Many people in the world don't really like what they are doing to earn their daily bread. Deep inside their hearts they want to quit it and start with something they love or always dreamed of. What stops them is the financial security aspect.

So, if there is some such hidden wish inside you then secondary source of income can be conceived from that wish of yours. You can get into share market and start investing. Markets are not your cup of tea, then how about pursuing the artistic side. If you love photography, try getting some proficiency in it and start a side biz covering your friend's wedding or selling your snaps. Painting, writing, reviewing for restaurants or movies are also viable options. How about home schooling service to children or even tuition on weekends? You can even start catering services. Learn yoga or aerobics and start your own morning classes. That way you stay fit and earn money too.